I always dreamed of a champion that releases their full power. Riven is an example, however to put it simply, she just reforms her broken sword and then magically gains a wind slash.
So my thought was the champion is gonna be weak levels 1-5. Then, once he (I'd rather have a male...currently, there's no male human that releases his full power. Yi doesn't really count) gets his ult, he can "break his chains" temporarily and gain new abilities momentarily, along with a "Final Ultimate" that depends on how much rage he builds up during his ult duration. Once he expends the Final Ultimate or outlasts the duration, he transforms back to his normal self. So it's kinda like a temporary transformation of Jayce/Nidalee/Elise, with no switch between melee/ranged. Then at levels 11 and 16, his transformation is even more powerful. (Think of PFE)
Although, that does sort of sound like Dragon Ball Z. (Ex. Saiyan turns Super Saiyan X, then uses most powerful move).
EDIT: For his normal form, he has more supportish abilities. While in his Final Form, he has more "caster" abilities.